It is often desirable to diffuse an element such as Si or Cr into the surface of iron or an iron alloy to improve its physical and/or chemical properties. It is known that if an element in its uncombined state is exposed in direct contact with another metal or alloy it will diffuse into that alloy. To hasten the diffusion, the temperature is raised. If that temperature exceeds the melting point of the element in question, or if the surface composition during diffusion exceeds the melting point, a liquid phase will form and disrupt the surface. It is desirable that the diffusion be accomplished as rapidly as possible for many purposes, without the hazard of molten phases. At high temperatures, when elements interdiffuse, various intermediate phases can form. These are predicted by the binary phase diagrams, many of which are well known.
For certain specific purposes, such as for electrical steels, it is undesirable to have intermetallic phases at the surface, as a consequence of the interdiffusion. Some inventors have interdiffused silicon into the steel surface by elemental contact, and suggest that the intermetallics can be removed easily by brushing. A far better situation is one in which the intermetallics are not allowed to form by controlling which compositions can form.
The object of this invention is to conduct the diffusion at high temperature and disallow melting by controlling the surface composition to values where molten phases are not produced.
Another object is to disallow formation of intermetallic phases. These phases are particularly undesirable for electrical steels. A further object of this invention is to rapidly interdiffuse silicon into low carbon steel to levels beyond the fabricability limit of 4.5% Si. Levels of 7% have specific magnetic properties, and cannot be made except by prior fabrication into the desired shape and subsequently introducing the desired silicon.